The present invention relates to a method for preparing fluorinated sulfinates and the use of the sulfinates obtained from the method as polymerization initiators, and particularly as initiators for the free radical polymerization of fluorinated monomers.
Fluorinated sulfinates have utility as reactive intermediates and precursors for fluorochemical sulfonyl halides and perhalofluorocarboxylic acids and their esters. They are also useful as initiators in free radical polymerization reactions.
For example, U.S. Pat. No. 5,285,002 describes the use of fluoroalkyl sulfinates as a source of fluoroalkyl radicals in aqueous emulsion polymerizations. The resulting polymers contain a high level of perfluoroalkyl end groups, and when fluoroalkyl disulfinates are utilized, fluoroalkyl moieties are incorporated into the polymer backbone thereby yielding fluoropolymers with specific microstructural fragments which are derived from the fluoroalkyl disulfinate.
U.S. Pat. No. 5,639,837 discloses a process in which fluorine containing olefins are polymerized using an initiation system which is a combination of a fluoroaliphatic sulfinate or sulfinic acid and chlorate, bromate or hypochlorite ions. The resulting polymer contains fewer deleterious end groups and is more stable and/or easier to process.
Methods for the synthesis of fluorochemical sulfinates and their use as intermediates have been widely reported in the literature. For example, perfluoroalkane sulfinates can be prepared from the corresponding perfluoroalkanehalides via a dehalogenation and sulfination reaction, as reported in C. M. Hu, F. L. Quing, and W. Y. Huang, J Org Chem, 1991, 2801-2804 and W. Y. Huang, Journal of Fluorine Chemistry, 58, 1992, 1-8. Several reagent systems have been developed for use in this reaction, such as sulfite plus an oxidant, hydroxymethane sulfinate, thiourea dioxide and sodium dithionite. The use of sodium dithionite as dehalogenating and sulfinating reagent has also been reported by W. Y. Huang, B. N. Huang and W. Wang in Acta Chim. Sinica (Engl. Ed.), 1986, 178-184, and Acta Chim. Sinica (Engl. Ed.), 1986, 68-72. The later publication discloses that the reaction with an aqueous solution of the sodium dithionite is too slow for reactions involving water-insoluble perfluoralkyl bromides, and that cosolvents are needed to improve the mutual solubility of the various reactants and permit completion of the reaction within 30 to 35 hours. Mentioned cosolvents include acetonitrile, glycol and diethyleneglycol.
F. H. Wu and B. N. Huang, Journal of Fluorine Chem, 67, 1994, 233-234 reported that if DMF, acetonitrile or alcohols are used as cosolvent, both polyfluoroalkyl iodides and polyfluoroalkyl bromides will react with sodium disulfite in neutral aqueous solution to give the corresponding sulfinates in good yield. In a similar manner, CF3CCl3 reacts with sodium disulfite to give the corresponding sodium sulfinate. A disadvantage of preparing fluorinated sulfinates starting from the corresponding fluorinated iodide or bromide is that the resulting reaction product contains a large amount of by-products, particularly, inorganic salts which typically must be removed from the sulfinate.
U.S. Pat. No. 3,420,877 describes an alternative process for the preparation of fluorocarbon sulfinates. The preparation involves reacting perfluoroalkyl sulfonyl fluoride with an alkali metal sulfite or alkaline earth sulfite in an aqueous medium containing from about 10 to about 50 weight percent of a dissolved polar, inert organic solvent selected from the group consisting of dioxane, dimethoxyethane, di-n-butyl ether, tetrahydrofuran, and diethylene glycol diethyl ether. This process generally does not result in large amounts of salts that need to be removed from the resultant product, but requires use of a cosolvent that may be toxic and may have a negative impact on processes in which the sulfinate is ultimately employed, e.g., free-radical polymerization reactions.
Accordingly, there continues to be a need for an improved process for preparing fluorinated sulfinates that does not require the use of toxic solvents and preferably does not require further processing or purification of the resulting reaction mixture. It is further desirable to improve the yield of the fluorinated sulfinate.
The invention provides a process for preparing a fluorinated sulfinate product which comprises the steps of:
a) providing a reaction mixture comprising:
(1) water;
(2) dehalogenating and sulfinating reagent; and
(3) fluoroaliphatic sulfonyl fluoride represented by the formula
(I): Rfxe2x80x94[SO2F]nxe2x80x83xe2x80x83(I)
wherein:
Rf represents a fluoroaliphatic group, and
n is a number of 1 to 4, and typically is 1 or 2; and
(4) at least one of the materials selected from the group consisting of (i) fluorinated surfactant, and (ii) organic cosolvent comprising, as the major component by weight of the organic solvent, alcohol having a boiling point of less than 110xc2x0 C. at 760 torr;
b) heating the reaction mixture to a temperature between about 50xc2x0 C. and 100xc2x0 C. until the desired product yield is achieved; and
c) where the organic cosolvent is employed in the reaction, optionally removing residual organic cosolvent from the resultant reaction mixture to achieve the desired level of product purity.
In a preferred embodiment, the invention provides a process for preparing a fluorinated sulfinate product comprising reacting in water, at a temperature between about 50xc2x0 C. and 100xc2x0 C.: (a) the fluoroaliphatic sulfonyl fluoride represented by formula (I) described above; with (b) a dehalogenating and sulfinating reagent; in the presence of the fluorinated surfactant, and without organic cosolvent until the desired yield of the product is achieved.
In another aspect, the invention provides a method of using the fluorinated sulfinate product obtained by the above-described processes as an initiation system for a free radical polymerization process.
In a preferred embodiment, the method may be used to prepare fluorine-containing polymers and comprises polymerizing, under free radical conditions, an aqueous emulsion or suspension of a polymerizable mixture containing the fluorinated sulfinate product and an oxidizing agent capable of oxidizing said tne fluorinated sulfinate compounds in the fluorinated sulfinate product to their corresponding sulfonyl radicals.
The invention provides a process for the preparation of fluorinated sulfinates using fluoroaliphatic sulfonyl fluoride that reduces or eliminates the need for organic cosolvents (such as dioxane, acetonitrile or 1,2-dimethyoxyethane) that may be toxic and/or interfere in reactions in which the fluorinated sulfinate product is used as a precursor, intermediate or initiator thereby eliminating or reducing the need for subsequent purification of the fluorinated sulfinate product. The invention accomplishes this improvement while providing comparable or improved yields to prior art processes.
Organic cosolvent may be used in the process either with or without fluorinated surfactant. The invention also includes reactions which do not use any organic cosolvent and employ only the fluorinated surfactant. The fluorosulfinate product largely comprises the fluorosulfinate derivatives of the fluoroaliphatic sulfonyl fluoride(s) used in the reaction mixture. Typically, the fluorosulfinate product will comprise at least about 50 weight percent and preferably at least about 70 or 80 weight percent fluorosulfinate compounds, based on the total weight of the fluorosulfinate product. The balance of the composition of the fluorosulfinate product is largely constituted of the sulfonate derivatives of the fluoroaliphatic sulfonyl fluoride(s) used to prepare the reaction mixture.
Representative fluoroaliphatic sulfinates that can be obtained with the process of the invention include the following:
CF3SO2Na
C8F17SO2Na
CF3C(Cl)2CF2 SO2K
Cl(CF2)8OC2F4SO2Na
NaO2SC8F16SO2Na
NaO2SC6F12SO2Na
NaO2SC2F4OC2F4SO2Na
NaO2SC2F4OC2F4X, where X is Br or I
(CF3)2NCF2CF2SO2Na
(C2F5)2NCF2CF2SO2Na
C4F9SO2Na
C6F13SO2Na.
The fluoroaliphatic sulfonyl fluorides useful in the invention may be represented by formula (I). The fluoroaliphatic sulfonyl fluoride may comprise a single fluoroaliphatic sulfonyl fluoride compound or a mixture of such compounds. The fluoroaliphatic radical, Rf, is a mono, or polyvalent (as provided in formula (I)), fluorinated, stable, inert, non-polar and saturated moiety. It can be straight chain, branched chain, and, if sufficiently large, cyclic, or combinations thereof, such as alkyl cycloaliphatic radicals. Generally, Rf will have about 1 to 20 carbon atoms, and preferably 3 to 10, and will contain about 40 to 78 weight percent fluorine, and preferably 50 to 78 weight percent fluorine. The preferred compounds are those in which the Rf radical is fully or substantially completely fluorinated, as in the case where Rf is perfluoroalkyl, CnF2n+1xe2x80x94, and n is 1 to 20. The skeletal chain of the fluoroaliphatic radical consists of carbon atoms and can be interrupted by divalent oxygen, hexavalent sulfur or trivalent nitrogen hetero atoms, each of which is bonded only to carbon atoms. However, where such hetero atoms are present it is preferred that the skeletal chain does not contain more than one hetero atom for every two carbon atoms. Where Rf is or contains a cyclic structure, the structure preferably has 6 ring member atoms, 1 or 2 of which can be hetero atoms, e.g., oxygen and/or nitrogen. Representative examples of Rf radicals include fluorinated alkyl radicals, e.g., C4F9xe2x80x94, C6F13xe2x80x94,C8 F17xe2x80x94, and alkoxyalkyl radicals such as, C3F7OCF2xe2x80x94.
The organic cosolvent comprises, as the major component by weight of the cosolvent, one or more alcohols having a boiling point of less than 110xc2x0 C. at 760 torr. In the context of this invention, the term xe2x80x9cmajor componentxe2x80x9d means at least about 50 weight percent of the organic cosolvent. Preferably, the organic cosolvent will comprise at least about 90 weight percent alcohol, more preferably at least about 99 weight percent alcohol, and most preferably will consist only of alcohol. Where the organic cosolvent does not consist only of alcohol, the remaining portion of the cosolvent can be constituted of any organic solvents suitable for use as a cosolvent to prepare fluorosulfinate compounds in aqueous reaction media. Many such organic solvents are described in the prior art. Lower alkanols, particularly those having between about 1 to 4 carbon atoms, are preferred for use in the process as organic cosolvents. Examples of useful alcohols include methanol, ethanol, isopropanol, n-butanol, tertiary butanol and isobutanol. Particularly preferred alcohols are ethanol and isopropanol. If the process is carried out in the presence of organic cosolvent, regardless of its alcohol content, the organic cosolvent will preferably be removed after completion of the reaction to achieve the desired level of fluorosulfinate product purity. Any conventional method may be used to remove the organic cosolvent, such as distillation under reduced pressure.
In a more preferred embodiment of the invention, the process is carried out in the presence of a fluorinated surfactant without any organic cosolvent. In this embodiment, it will not be necessary to remove organic cosolvent from the fluorinated sulfinate product after completion of the reaction, and the resulting product may be used directly as an initiator in free radical polymerization processes without further purification. Moreover, where the fluorinated sulfinate product is intended for use in an initiation system in an emulsion or suspension polymerization process, the fluorinated surfactant used to make the fluorosulfinate product can be the same as the fluorinated surfactant employed in the polymerization process, thereby providing additional polymerization reaction efficiencies. However, it is preferred not to select a fluorinated surfactant which is the same as the fluorosulfonate derivative of the fluoroaliphatic sulfonyl fluoride (i.e., Rfxe2x80x94[SO3X]n, where n is as defined above and X is a cation such as Na) used in the reaction mixture.
Also, the fluorinated surfactant preferably will be present in the reaction mixture prior to heating the reaction mixture to ensure that the fluorinated surfactant is present from the beginning of the reaction to make the fluorinated sulfinate product.
Representative fluorinated surfactant useful in the invention may be represented by formula (II):
R1fxe2x80x94Qxe2x80x94Zxe2x80x83xe2x80x83(II)
wherein:
R1f represents a fluoroaliphatic group,
Q represents an organic linking group or a covalent bond, and
Z represents an ionic group or a water solubilizing group containing a poly(oxyalkylene) group.
Examples of ionic Z groups include groups having the formula, T-X, where the T, the surfactant polar moiety, is linked via Q to R1f, and X is a counterion to T. Examples T-X groups include those wherein T is a carboxylate group (such as ammonium carboxylates, sodium carboxylates and potassium carboxylates), those wherein T is a sulfonate group (such as ammonium sulfonates, sodium sulfonates and potassium sulfonates) and those wherein T is an ammonium group (such as in ammonium chlorides and ammonium iodides). Z can also be a water solubilizing group containing a poly(oxyalkylene) group, (ORxe2x80x2)s, where Rxe2x80x2 preferably is an alkylene group having 2 to 4 carbon atoms, such as xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94CH(CH3)CH2xe2x80x94, and xe2x80x94CH(CH3)CH(CH3)xe2x80x94 or mixtures thereof, and s preferably is a number from about 4 to about 25, and more preferably from about 6 to 20. The oxyalkylene units in the poly(oxyalkylene) group may be the same, as in poly(oxypropylene), or present as a mixture, such as in a heteric straight or branched chain of randomly distributed oxyethylene and oxypropylene units (e.g., poly(oxyethylene-co-oxypropylene)), or as in a straight or branched chain of blocks of oxyethylene units and blocks of oxypropylene units. The poly(oxyalkylene) chain can be interrupted by or include one or more catenary linkages, providing such linkages do not substantially alter the water-solubilizing character of the Z-group. The Z group may terminate with a hydrogen or a lower alkyl ether. Most preferably, Z represents an ionic group T-X wherein T represents an anion.
R1f is a fluoroaliphatic group and is typically a fluorinated, stable, inert, non-polar, saturated moiety. Examples of suitable R1f moieties are those present in the compounds described below. Preferably, R1f contains at least 5 carbons and most preferably, R1f is a perfluorinated group. Specific examples of fluorinated surfactants useful in the invention include:
C7F15COONH4 
C6F13SO3NH4 
C3F7OCF(CF3)CF2OCF(CF3)COONH4 
C7F15CF(CF3)COONH4 
C8F17SO2N(CH2CH3)CH2CH2(OCH2CH2)iOCH3; where i is approximately 8
C8F17SO2NH(CH2)3N(CH3)3I
The amount of fluorinated surfactant added to the reaction mixture is not particularly critical. Preferably, the amount of fluorinated surfactant used will be between about 0.01 to 10% by weight of the total amount of fluoroaliphatic sulfonyl fluoride used to prepare the reaction mixture. If less than about 0.01% of the surfactant is employed in the reaction, the reaction will be very slow, particularly at the beginning. If more than about 10% of the surfactant is used, foaming of the reaction mixture may occur.
Examples of dehalogenating and sulfinating agents which are useful in the invention include those known in the art such as inorganic sulfites (e.g., alkali metal sulfites and earth alkali metal sulfites), inorganic dithionites (e.g., sodium dithionite and potassium dithionite) and hydrazine and certain metals, such as zinc. Particularly preferred dehalogenating and sulfinating agents are alkali metal sulfites or alkaline earth metal sulfites and specific examples of such sulfites include sodium sulfite and potassium sulfite.
The reaction may further be carried out in the presence of fluoride capturing agents such as an inorganic hydroxide, carbonate or bicarbonate compounds. Particularly useful fluoride capturing agents include alkali metal and alkaline earth metal carbonates or bicarbonates.
The reaction is carried out at a temperature between about 50xc2x0 C. and 100xc2x0 C. At temperatures below 50xc2x0 C., the reaction will be slow whereas at a too high temperature, decomposition of the sulfinates in the product may occur. A preferred temperature range is between 55xc2x0 C. and 85xc2x0 C. and more preferably between 60xc2x0 C. and 80xc2x0 C.
The fluorinated sulfinate products prepared in accordance with the process of this invention are particularly suitable for initiating a free radical polymerization of ethylenically unsaturated monomers. Preferably, the fluorinated sulfinate products are used to initiate the homo- or copolymerization of polymerizable mixtures comprising fluorine-containing ethylenically unsaturated monomer, and optionally, fluorine-free, terminally unsaturated monoolefin comonomers (e.g., ethylene or propylene), or iodine- or bromine-containing cure-site comonomers. The polymerization technique used is typically an emulsion or suspension polymerization in an aqueous medium.
Examples of fluorine-containing ethylenically unsaturated monomers include the terminally unsaturated monoolefins typically used for the preparation of fluorine-containing polymers such as vinylidene fluoride, hexafluoropropene, chlorotrifluoroethylene, 2-chloropentafluoropropene, perfluoroalkyl vinyl ethers (e.g., CF3OCFxe2x95x90CF2 or CF3CF2OCFxe2x95x90CF2), tetrafluoroethylene, 1-hydropentafluoropropene, 2-hydropentafluoropropene, dichlorodifluoroethylene, trifluoroethylene, 1,1-dichlorofluoroethylene, vinyl fluoride, and mixtures thereof. Perfluoro-1,3-dioxoles may also be used. The perfluoro-1,3-dioxole monomers and their copolymers are described, for example, in U.S. Pat. No. 4,558,141 (Squire). Certain fluorine-containing di-olefins are also useful, such as, perfluorodiallylether and perfluoro-1,3-butadiene.
Examples of iodine- or bromine-containing cure-site comonomers include those known in the art for the preparation of peroxide-curable polymers, e.g., peroxide-curable fluoroelastomers. Suitable cure-site monomers include terminally unsaturated monoolefins having about 2 to 4 carbon atoms such as bromodifluoroethylene, bromotrifluoroethylene, iodotrifluoroethylene, and 4-bromo-3,3,4,4-tetrafluorobutene-1.
The fluorine-containing monomer may also be copolymerized with fluorine-free terminally unsaturated monoolefin comonomers, e.g., ethylene or propylene. However, preferably at least 5% by weight, and most preferably at least 50%, of all monomers in the polymerizable mixture are fluorine-containing. Preferably, all or essentially all of the comonomers in the polymerizable mixture are ethylenically unsaturated monomers.
The initiation of a free radical polymerization in accordance with the invention uses the fluorinated sulfinate product as a reducing agent and a water-soluble oxidizing agent capable of converting the sulfinate compounds in the product to their corresponding sulfonyl radicals. Preferred oxidizing agents are sodium, potassium, and ammonium persulfates, perphosphates, perborates, and percarbonates. Particularly preferred oxidizing agents are sodium, potassium, and ammonium persulfates. Still further suitable oxidizing agents include water-soluble oxidizing agents containing a chlorate ion, a bromate ion or a hypochlorite ion. The sulfonyl radical so produced is believed to eliminate SO2, forming a fluorinated radical that initiates the polymerization of the ethylenically unsaturated monomers.
In addition to the fluorosulfinate product, other reducing agents can be present in the polymerizable mixture, such as sodium, potassium or ammonium sulfites, bisulfite, metabisulfite, hyposulfite, thiosulfite, phosphite and sodium or potassium formaldehyde sulfoxylate or hypophosphite. Activators such as ferrous, cuprous, and silver salts, may also be present.
Aqueous emulsion polymerizations can be carried out under conventional steady-state conditions in which, for example, monomers, water, surfactants, buffers and catalysts are fed continuously to a stirred reactor under optimum pressure and temperature conditions while the resulting emulsion or suspension is removed continuously. An alternative technique is batch or semi-batch polymerization by feeding the ingredients into a stirred reactor and allowing them to react at a set temperature for a specified length of time or by charging ingredients into the reactor and feeding the monomer into the reactor to maintain a constant pressure until a desired amount of polymer is formed.
The amount of fluoroaliphatic sulfinate product used in such polymerization reactions can vary, depending, for example, on the molecular weight of polymer desired. Preferably the amount of fluoroaliphatic sulfinate product used is from about 0.01 to 50 mole %, and most preferably 0.05 to 10 mole %, based on total quantity of monomer in the polymerizable mixture.
Combinations of monosulfinates, disulfinates, and trisulfinates can be used, depending on whether it is desired to use sulfinate as an initiator, a monomer, or both. When polyvalent sulfinates are used, the sulfinate segment is incorporated into the polymer backbone. When monosulfinates are used, the fluorinated moiety is incorporated as a polymer end group.